Water-soluble polyacrylamide (PAM) and other water-soluble polymers are used as soil conditioners because they help form and protect soil aggregates by binding to clay particles in the soil. Among the benefits, this characteristic helps to control wind and water erosion, improve water infiltration and retention, improve soil aeration, and inhibit crusting or sealing. One use is in forestry, wherein granules of PAM are mixed into soil into which seedlings are planted. Water-soluble PAM is a long-chain molecule, which is distinguished from an insoluble cross-linked form of PAM. This disclosure relates to the water-soluble variety of PAM, which will also be referred to herein simply as PAM.
Water-soluble PAM with the most desirable properties for soil conditioning has a molecular weight of about 15-22 million a.u. (atomic units), and is about 20% anionic. Water-soluble PAM of this molecular size is commercially available in granule sizes of about +60, −25 mesh (between 250 and 600 μm), and, more typically, essentially not smaller than +40 mesh (>450 μm) sizes. PAM of this type, which comprises an anionic linear copolymer of acrylamide and sodium acrylate, is available under the trade name FLOBOND A30 from Chemtall, Inc. of Riceboro, Ga. The water-soluble granules appear to be aggregates of many molecules and have a very irregular shape. When viewed under an optical microscope, flat crystal faces are visible, and the particles appear translucent. Some small portions of the granules appear darkly transparent. The granules are probably not entirely clear because of included fractures and other defects, which will scatter light.
Severe barriers exist to more widespread use of water-soluble PAM in soil. When dry granules of PAM of standard commercial sizes are applied to agricultural soil at reasonable economic rates, the soil typically is nonuniformly conditioned because the dry granules are too far apart to fully condition soil. For example, when 10 pounds (4.5 kg) of the standard size water-soluble PAM is mixed into an acre of soil to a uniform depth of six inches (15 cm), each pound (0.45 kg) of soil will contain on average only about 22 particles of water-soluble PAM. Each ounce (28.4 g) of soil would contain only about 1-2 particles.
Applying water-soluble PAM in solution is more effective than dry granule application because it produces a more uniform spatial distribution of the PAM in the soil. Therefore, solution application requires less PAM than does application of granules for favorable results. Another benefit of using a solution of PAM is that it can be applied to soil through standard irrigation lines. Full solution time for standard size granules is typically an hour or more, but many dealers and manufacturers state that it is best to allow the water-soluble PAM to sit overnight in water to fully dissolve for most large scale agricultural purposes.
Generally, stock solutions of around 2,000 to 3,000 parts per million (ppm) by weight (mg/liter) water-soluble PAM in water are the most concentrated that can be conveniently obtained with conventional procedures. When some fertilizer salts are included, somewhat higher concentrations, e.g., up to about 12,000 ppm are possible. The presence of fertilizer solutes will also help PAM go into solution somewhat more rapidly. For example, solution concentration can be improved by dissolving PAM in various fertilizer salt solutions, as described in U.S. Pat. No. 4,797,145, to Wallace, et al. However, the solution rate generally remains slower than desirable because of the large particle sizes. An hour or more is typically required to achieve the higher concentrations even with the use of fertilizer salts.
Co-application of dry PAM with some divalent calcium helps the water-soluble PAM to bridge with or react with clay to add stability to soil aggregates, as described in Need for Solution or Exchangeable Calcium For Critical EC Level for Flocculation of Clay by Polyacrylamides, by Wallace and Wallace, in Proceedings: Managing Irrigation Induced Erosion and Infiltration with Polyacrylamide, Univ. Idaho Misc. Pub. No. 101-96, pp. 59-63, 1996. This enhances the soil conditioning value. To achieve this, solution-grade gypsum of −200 mesh particle size (−75 μm), which is typically composed mostly calcium sulfate dihydrate, can be applied to soil before adding water-soluble PAM, or gypsum and water-soluble PAM can be applied together in solution after each is dissolved separately. But gypsum also has a low solubility, which limits its use.
Using water-soluble PAM in solution for liquid application to soil entails high handling costs. Because of the low solubility of PAM, large quantities of stock solution are usually required to apply a reasonable amount of PAM to a large field. It generally requires bulk equipment that is not easily portable to fields. The PAM granules in commercially available PAM take a long time (anywhere from over an hour to overnight) to go into solution. Because low PAM concentrations of stock solutions are the rule, large amounts of stock solution are needed for each application. Dissolving the standard commercial-size polymers requires considerable experience. Training is often required to become proficient in getting water-soluble PAM particles into solution. When directions are not followed accurately, failed applications result. If the PAM is not completely dissolved, particles of PAM tend to clump together in agglomerations. These large, undissolved clumps make using solutions of PAM in sprinkler irrigation systems very difficult, if not impossible, as the clumps tend to clog the sprinkler lines and nozzles.
One approach to the time and concentration problems has been to use mechanical devices that meter the water-soluble PAM into a stream of irrigation water. A residence time in a tank of one hour or more to achieve solution before applying the solution to fields is realistic and common. For example, U.S. Pat. No. 5,450,985, to Meuleman, discloses a device that delivers dry water-soluble PAM into a canister and from there into an irrigation water stream, such as an irrigation ditch or canal, after a time period. This system does not produce PAM solutions for injection into sprinkler systems. U.S. Pat. No. 5,580,168, to Alireza, et al., discloses a venturi system for injecting water-soluble PAM first into a dispersion tank and then into an aging tank, which is further agitated before injecting the stock solution into an irrigation system. Solution time for both systems is over an hour, and much too slow for convenience. The size of the granules of Water-soluble PAM that are used commercially is too large to allow faster solution times.
U.S. Pat. No. 5,548,020, to Santini et al., discloses an alternative procedure for putting water-soluble PAM into irrigation lines. A 30% concentrated PAM emulsion product is prepared with kerosene or oil. The flow rate of this product is relatively slow, which decreases its usefulness for sprinkler irrigation systems. Also, this form of PAM emulsion flows into water like a semi-stiff string that requires considerable mechanical agitation by a machine to put into solution. The kerosene or oil is environmentally undesirable. The kerosene or oil adds expense and appears to decrease the effectiveness of the PAM as a soil conditioner, thereby requiring more PAM to be used for the same effect as with an aqueous solution. The water-soluble PAM in the 30% emulsion is considerable more expensive than granular forms of water-soluble PAM to further detract from any advantage it may have.
Water-soluble polymers also have uses in the oil drilling industry. Oil drillers have developed various methods for putting the polymers into solutions. The polymer solutions produced for oil drilling have a high viscosity, which is undesirable for irrigation. The oil drilling solutions are typically produced without any accurate measuring of the amounts of the ingredients, which is not appropriate for agriculture. For these reasons, the oil drilling industry methods are not applicable to the aqueous requirements for solution of water-soluble APM for agricultural use.
It would seem that the time needed for dissolution of water-soluble PAM should be decreased by using smaller particle size PAM however, there is no adequate bulk supply of suitable small particle PAM. Attempts have been made to grind PAM into smaller sizes, such as −100 mesh (<150 μm). However, PAM ground in this way loses many of its desirable properties. For example, the ground PAM flows very poorly, clumping up even when dry. This characteristic is undesirable in a metering system. The poor flow and clumping is probably because the grinding has made the PAM irregularly shaped. When observed with an optical microscope, the particles are entirely opaque. Most particles do not appear to have clean, flat faces, as with the larger, commercial grade particles. These ground particles also tend to form clumps in water, which are difficult to dissolve. Many particles remain visible and undissolved in water even after an hour or more. The solution formed from the dissolved particles is less viscous than a solution produced with a like concentration of PAM that was not ground. This suggests that the grinding process has broken many of the large polymeric molecules. Analysis of the ground particles confirms that the molecular weights are reduced to less than one million a.u., and typically less than a few hundred thousand a.u. Such smaller-chain particles are not as effective for soil conditioning, and so more must be used to achieve the same result. In addition, the known process for grinding the PAM granules includes freezing the granules, which adds to the cost and complexity of the procedure.
Small quantities of small particle size water-soluble PAM can be obtained by screening the generally large-granule PAM from commercial sources. This is a laborious process that does not yield sufficient quantities to be commercially cost effective. The screened fines are very similar in appearance to the larger size, commercial grade PAM. When observed with an optical microscope, the particles are translucent, which may be due to fractures included in the particles. About 40-60% of the particles appears to have areas that are transparent. When added to plain water, the particles do not disperse or dissolve well. Some of the particles appear to clump together. Many of the particles remain undissolved and visible, even after ten minutes of stirring or agitation. The screened fines also have poor flow characteristics when dry.